1. Technical Field
The present disclosure relates generally to a data access control method in cloud computing and, more particularly, to a data access control method using Ciphertext Policy-Attribute Based Encryption (CP-ABE).
2. Description of the Related Art
A cloud computing system (hereinafter referred to as a “cloud”) is a new innovative model of distributed computing that is composed of a centralized data center.
A cloud provides high expandability and stability. That is, client devices having limited resources delegate (offload) a required data storage space and the function of computing corresponding data to a cloud. Through this, data itself or the result of the computation of the corresponding data stored in the cloud may be shared by other users. Accordingly, users can use a high-capacity and highly-efficient computing function at low cost.
However, since the data of users is stored, processed and shared within a cloud, measures for security and privacy protection should be taken. That is, since the cloud can access stored user data without the intervention of the users, measures for privacy protection should be taken.
To perform such privacy protection, a data owner may encrypt and store data to be uploaded to the cloud. Furthermore, to control access to data stored via encryption, recently, Key Policy (KP)-Attribute-Based Encryption (ABE) and Ciphertext-Policy (CP)-ABE to which ABE has been extended are widely being used.
ABE is a public key cryptographic algorithm to which IBE, which uses the identification information of an entity as the public key of a corresponding entity without using a certificate, has been extended. According to ABE, data encrypted using a specific attribute set as a public key may be decrypted using an entity having an attribute set whose intersection with respect to a corresponding attribute set exceeds a specific threshold value. ABE operates based on a bilinear group and a bilinear map.
In the 2000s, KP-ABE and CP-ABE, i.e., extended versions of ABE, were proposed in 2006 and 2007, respectively. Various methods of controlling access to the data of a cloud have been proposed.
Data access policies that are described by KP-ABE and CP-ABE in common are represented by an access tree composed of leaf nodes representative of attributes and intermediate nodes representative of AND or OR gates. In KP-ABE, the result of the encryption of specific data is represented by a specific attribute set, and an access tree is applied to a key used to decrypt the corresponding encrypted data. In contrast, in CP-ABE, an access tree is used to encrypt specific data and an attribute set is used to represent a key used to decrypt the corresponding encryption data.
That is, in KP-ABE, a single piece of encrypted data is decrypted using various scenarios and users. In contrast, in CP-ABE, a single user can decrypt various pieces of data according to his or her attribute.
As a related technology, “Achieving Secure, Scalable, and Fine-grained Data Access Control in Cloud Computing” (hereinafter referred to as “related technology 1”) published on IEEE INFOCOM in 2010 discloses a technology in which a data owner encrypts a data file with a Data Encryption Key (DEK). Furthermore, related technology 1 adds the result of the KP-ABE encryption of a DEK and the ID of the data file to the header of the encrypted data file and stores the resulting data in a cloud. Furthermore, in related technology 1, whenever a user attempts to access a corresponding encryption file, a data owner generates and transfers a secret key that is used when a corresponding user decrypts a DEK encrypted through KP-ABE. Furthermore, in related technology 1, a user obtains a DEK by performing KP-ABE decryption of the DEK using a received secret key, and then decrypts a data file. Additionally, in related technology 1, upon the revocation of a user, the result of the KP-ABE encryption of a DEK accessed by the corresponding user is delegated to a cloud via a re-encryption proxy method, thereby reducing the computational overhead of the data owner.
However, since a DEK itself is not updated upon the revocation of a user, data files encrypted with the same DEK and stored for a long period may be vulnerable in terms of security. Furthermore, a vulnerable point in which, when the user subjected to revocation colludes with the cloud, the user can recover the DEK encrypted through KP-ABE. Furthermore, whenever a new user is added, the data owner should generate and transmit a secret key for the corresponding user, and thus overhead is considerably large.
As another related technology, “Hybrid Attribute- and Re-Encryption-Based Key Management for Secure and Scalable Mobile Applications in Clouds” (hereinafter referred to as “related technology 2”) published on IEEE Transaction on Cloud Computing in 2013 employs CP-ABE. In related technology 2, a secret key generation process for a user that is performed by a data owner in the case of related technology 1 is performed by the data owner and a manager, i.e., a reliable entity, and pairing computation that causes considerable overhead in CP-ABE computation is performed by the manager and a cloud, with the result that related technology 2 is appropriate to a mobile cloud computing (MCC) environment. Furthermore, in related technology 2, whether to access data can be determined based on the possession of a specific attribute or belonging to a specific group via a group key. Based on this, in related technology 2, when a user who belongs to a specific group is subjected to revocation, a corresponding group key is updated, and data encrypted with a previous group key is encrypted with a group key updated by the cloud through proxy re-encryption.
However, related technology 2 has limitations in that the update of a DEK itself is not addressed therein and the target of the update of a CP-ABE encryption result attributable to the revocation of a user is limited in connection with belonging to a specific group.
As still another related technology, Korean Patent No 1220160 (entitled “Safe Proxy Re-encryption-based Data Management Method in Mobile Cloud Environment”) (hereinafter referred to as “related technology 3”) discloses a technology that performs data access control in a cloud environment in the same structure as above-described related technology 1. In this case, in related technology 3, a data owner transmits a file header, including the result of the encryption of a DEK using KP-ABE, to an authority administrator. Furthermore, a data file body encrypted with a corresponding DEK is transmitted to a cloud, and the header and body of an encrypted data file are separated and managed. Furthermore, in related technology 3, secret distribution and type-based proxy re-encryption are used to verify the legitimacy of user access. Through this, in related technology 3, the vulnerability to a collusion attack in the above-described related technology 1 is mitigated. However, related technology 3 does not overcome the issue of the overhead of a data owner, attributable to the generation of a KP-ABE secret key, and the issue of the update of a DEK itself.